The application of technology known as multiple-input multiple-output (MIMO) has become prevalent for wireless communication in recent years. MIMO uses multiple transmitters and multiple receivers all operating in the same radio frequency. Modern wireless standards such as Long Term Evolution Advanced (LTE-Advanced) mobile standard and IEEE 802.11ac wireless local area network standard define the use of eight or more transmitters for the purpose of MIMO wireless communication.
Typically, the application of MIMO requires the estimation of radio propagation channels between a single receiver and multiple transmitters. A conventional method to achieve this is to use pre-determined symbols known both to the transmitter and the receiver, herein called channel training symbols.
In IEEE 802.11ac, code orthogonal channel training symbols are used so that the radio propagation channel specific to each transmitter (or transmit stream) can be distinguished and estimated by simple flipping of sign and summation of received symbols. The summation of received symbols allows the reduction of channel estimation error by reducing the effects of random receiver noise. The advantage of the reduction of channel estimation error by this method is herein called the processing gain. However, this operation is effective only if the radio propagation channels during the channel estimation period can be considered stationary. In the case of IEEE 802.11ac, the maximum channel training period corresponds to 4 μsec×8=32 μs, and hence the method is expected to work well as long as the variation of the radio propagation channels within 32 μs is small.
The same method may not be applicable when the symbol time becomes longer and/or the variation of the radio channel within the channel training symbol time becomes larger, such as in the case of LTE-Advanced mobile standard, where one symbol time is 66.7 μs or longer. In LTE-Advanced mobile standard, the assumption of stationary channel over eight or more symbol time may not be valid. Alternatively, LTE-Advanced mobile standard defines the transmission of channel training symbol from only one transmitter within any one time-frequency slot. The radio propagation channel can be estimated without interference from other transmitters in this case. The advantage of this method is to be able to cope with a fast time varying channel. However, disadvantage of this method is the loss of processing gain to reduce the effects of receiver noise at the time of receiving the channel training symbols.
There is therefore a need to estimate the time varying channel for multiple of transmitters while taking advantage of processing gain as well as being able to cope with a fast time varying channel.
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